1. Field of the Invention
This invention refers to the composition of a flame-resistant photo-curable resin mixture, more specifically, a pattern-forming flame-resistant resin mixture that can be used as a solder resist (solder mask) in printed circuit board (referred to as "PC-board" below) manufacturing.
2. Prior Art
In the PC-board manufacturing industry, the solder resist (solder mask) is widely used as a permanent protective film on PC boards. Solder resist is used to prevent solder bridges that can form during soldering, and to prevent conductor corrosion and protect electric insulation during use. Since the solder resist must be able to withstand severe process and usage conditions, it must be capable of satisfying the following performance requirements:
Ability to maintain adhesion during immersion in solder (240.degree.-280.degree. C.) PA1 Ability to maintain permanent adhesion PA1 High resistance to solvents and other chemicals PA1 Ability to maintain high electric insulation in a high-humidity environment PA1 Flame resistance PA1 A represents ##STR4## wherein R.sub.2 is an alkylene group containing two or three carbon atoms, and n is an integer between 1 to 4; and PA1 X.sub.1, X.sub.2, and X.sub.3 are Br or H, under the condition that at least two of the three are Br. PA1 A is ##STR7## wherein R.sub.2 is an alkylene group containing two or three carbon atoms and PA1 n is an integer between 1 and 4; and PA1 X.sub.1, X.sub.2, and X.sub.3 are Br or H, under the condition that at least two of the three are Br.
To satisfy these requirements, heat- or photo-curable inks have widely been used together with screen printing methods to form solder resists. In recent years, however, these printing methods have not kept pace with advances in circuit complexity and circuit element size reductions, and these methods have therefore become unsuitable for recent high-density printing method with regard to precision and fineness of detail. The screen printing method has reached a limit with regard to accuracy because further extension of the screen mesh can cause positional deviations and the ink may bleed or become blurred. On the other hand, if it is used to forcibly enhance the precision, the film may become too thin. Thus, screen printing is incapable of providing both the necessary thickness together with high precision.
Therefore, in recent years, the photo resist method is replacing the screen printing method. In the photo resist method, a photo-curable film is formed on the board and selectively exposed to light for curing. Following exposure, the non-cured portions are removed in a developing process. This method is far superior to the screen printing method, especially with respect to precision.
Several proposals have been made so far with regard to the formation of the photo-curable film used in the photo resist method. Japanese Patent Application, first publication No. (Tokukai Sho) 51-15733 proposes a method in which a photo-curable resin is applied to a printed circuit board in the form of a solution in an organic solvent, and then heat is applied to evaporate the solvent, leaving behind the photo-resist film. In this method, however, small through-holes not used for component mounting cannot be tented and the film on a pattern cannot be made thick enough.
It is preferable to apply a specific dry film resist proposed in Japanese Patent Application, first publication No. (Tokukai Sho) 54-1018 by heating under low pressure, as proposed in Japanese Patent Application, first publication No. (Tokukai Sho) 52-52703. However there is no dry film resist which can be developed with an alkaline developer and also finally cured product has necessary properties as a solder resist such as high adhesion, surface hardness, heat resistance and flame resistance.
The aim of the present invention is to provide a photo-curable resin composition for solder resist that can be developed by an alkaline developer in the pattern forming process and that has excellent flame resistance and other characteristics.